Abstract

Nitric oxide (NO) is a short-lived intermediate in several processes of the oceanic nitrogen cycle, whereby denitrification is assumed to be the major pathway for NO in the ocean. Therefore, measurements of dissolved nitric oxide could be used to identify hot-spots of ongoing denitrification. Despite the fact that until today many laboratory studies about the role of nitric oxide in the nitrogen cycle have been performed, only a few depth profiles of dissolved nitric oxide from oceanic regions such as the eastern tropical North Pacific are available. One reason for this is the demanding analytical instrumentation necessary to detect the reactive and thus short-lived NO in seawater samples. To this end we set up a measurement system for dissolved NO which consists of a chemiluminescence analyzer connected with a stripping unit for discrete seawater samples. The first field test of this set up took place during a cruise near the coast of Peru in February 2013 (SFB 754 cruise M93). NO was measured at 40 stations in the oxygen minimum zone (OMZ) along 7 transects toward the coast. We found an accumulation of nitric oxide in oxygen depleted waters with the highest concentrations on the shelf and a decrease of nitric oxide concentrations towards the open ocean suggesting that denitrification rates on the shelf were higher than in the OMZ of the open ocean.